This invention relates generally to semiconductor devices and manufacturing methods and more particularly to semiconductor devices formed by vapor phase epitaxy.
As is known in the art, the performance of many semiconductor devices depends on the doping profile of materials from which the devices are manufactured. It is particularly important to the yield and cost of the semiconductor devices to be able to control doping levels of such materials. This is often accomplished by introducing a controlled quantity of a selected doping substance into the growth region of a vapor phase epitaxial reaction to dope layers of a semiconductor device as they are being deposited. In forming n-type gallium arsenide IMPATT diodes using vapor phase epitaxial deposition of an n-type gallium arsenide layer, for example, doping vapors, such as hydrogen sulfide or silane, are introduced into a reaction chamber along with materials including gallium and arsenic which produce the vapor phase epitaxial deposition of the layer of gallium arsenide. Here it is noted that these dopants are gases at room temperature and therefore doping concentrations may be controlled precisely using conventional gas handling equipment. To obtain relatively low doping concentrations of such n-type dopants proper mixing of the doping gas with an inactive gas, such as hydrogen or helium, may be used.
While such technique enables accurate and practical control of doping concentrations because the dopants are readily available in gaseous form, such technique is not practical for dopants which exist at room temperatures in solid or liquid form. More specifically, while such doping techniques may be used in the formation of n-type layers of gallium arsenide, such technique is not readily available in the formation of p-type layers of gallium arsenide because many p-type dopants for gallium arsenide, such as cadmium, zinc, beryllium, magnesium and manganese do not exist in gaseous form at room temperature. It follows, then, that while such vapor phase epitaxy deposition techniques may be used to form single drift n-type gallium arsenide IMPATT diodes, difficulties have been encountered in the formation of double drift gallium arsenide IMPATT diodes having both high quality n-type layers and high quality p-type layers of gallium arsenide.
One technique suggested to obtain p-type gallium arsenide layers is to provide metal organic compounds of zinc or cadmium, such as zinc alkyls, either dimethyl or diethyl zinc, which are liquid at room temperature with a sufficiently high vapor pressure to enable transport of surface vapors of such liquid by a carrier gas, such as hydrogen or helium, to the epitaxial reaction chamber along with the gases and vapors used to vapor phase epitaxially deposit a layer of gallium arsenide. The transported vapors are sometimes passed through a condenser prior to being introduced into the reaction chamber. The doping concentration of the deposited layer is controlled by: Controlling the quantity of the carrier gas over the surface of the metal organic compound; controlling the temperature of the metal organic compound to control the vapor pressure of the doping vapors; and, controlling the temperature of the condenser to control the amount of dopant which is introduced into the chamber. Such control is relatively imprecise and ineffective, particularly where doping concentrations of less than 10.sup.16 per cm.sup.3 are required.
Another technique suggested for providing p-type doped gallium arsenide layers during vapor phase epitaxial deposition is through the direct vapor vaporization of the dopant from its solid or liquid state. To achieve a controlled doping with such technique the doping material must generally be held at a very precise temperature, the flow of a carrier gas must be carefully regulated, and the temperature of all surfaces between the heated dopant and the substrate on which the layer is being deposited must be high enough to prevent dopant condensation. In addition, the dopant vapor must generally experience no, or little, uncontrolled reactions upstream of the substrate. For example, solid dopants which have been suggested for direct vaporization in providing p-type doped gallium arsenide layers by vapor phase epitaxy are cadmium and zinc. Here a small amount of the metal is heated with a metered flow of a carrier gas passing over the surface of the metal to carry sublimed vapors over such surface into the reactor. As noted, precautions must be taken to ensure that all parts of the vapor path between the zinc or cadmium and the substrate are held at temperatures above that of the dopant source; otherwise, condensation occurs and the doping is unpredictable. To change doping levels or to initiate or terminate doping the carrier gas flow is changed or the temperature of the metal is changed. Where it is desired to form doping spikes a large increase in the flow rate of the gas to provide such doping spikes adversely affects the growth of the doped layer. Further, rapid changes in metal temperature are not practical because of time lags. Consequently, such technique is not readily suitable where it is desired to sharply change doping concentrations as when it is desired to form a p-type doping spike having a background concentration near 3.times.10.sup.15 per cm.sup.3, a peak concentration near 10.sup.17 per cm.sup.3 and a width of several tenths of a micrometer. Further, when it is desired to form double drift devices it is generally desired to efficiently inhibit the flow of p-type dopant from entering the reactor during deposition of n-type layers and this is not practical with valves because such valves must operate in the high temperature environment between the heated metal and the reaction chamber, such high temperature environment being necessary to prevent condensation as mentioned previously above. Alternatively, where such high temperature valves are not used, n-type doping compensation techniques have been suggested; however, such techniques require the addition of additional impurity atoms, thereby reducing carrier mobility and adversely affecting the electrical characteristics of the device.
An alternative technique suggested to obtain p-type doped layers includes the use of a p-type doped source of gallium arsenide. Zinc, for example, is added to a gallium source. The source material is transported by passing arsenic trichloride over such material. The zinc transports preferentially to the gallium and, therefore, the first runs after the addition of zinc to the source are heavily doped, and subsequent runs are less p-type doped. After a few runs, no p-type doping is obtained.
In still another method suggested, the reactive gas used in the vapor phase epitaxy deposition is also used to react with a dopant. Such technique does not provide for independent control of both the reactive transport and the doping.
In other semiconductor devices, such as gallium arsenide field effect transistors, the importance of chromium doping in producing semi-insulating substrate material for the deposition or implantation of layers is well known. However, commercially available chromium-doped gallium arsenide substrates are generally of variable quality and strongly influence the performance of devices made upon them. Two approaches have been suggested: The first requires that potential substrates be subjected to elaborate qualification procedures prior to use. However, this is costly, and the second procedure isolates the substrate from the active layers by the intermediate deposition of a highly resistive buffer layer. The buffer layer restricts the diffusion of impurities from the substrate into the active layer, sharpening the interface profile, and improving values of carrier mobility at the active layer-buffer interface. On the average, wafers with buffer layers produce devices with improved noise figures and gain when compared with unbuffered wafers. Nevertheless, it has been found that the properties of undoped buffer layers still depend upon substrate characteristics. Efforts have been made to improve the buffer layer quality by deliberately adding dopants during deposition. Iron doping was used in an AsCl.sub.3 --N.sub.2 --Ga open tube vapor phase epitaxy reactor by passing hydrogen chloride, formed by the thermal decomposition of arsenic trichloride, over heated iron to form FeCl.sub.2 which was vaporized and carried into the reactor. This process, however, uses nitrogen as the carrier which is not generally compatible with forming silicon, n-type dopant layers as the active layer of the field effect device. Chromium dopant has been obtained by introducing vaporized chromyl chloride into an AsCl.sub.3 --H.sub.2 --Ga vapor phase epitaxy. Here chromyl chloride contains oxygen and reacts with hydrogen to form chromium oxide, much of which deposits on the heated walls of the dope tube. A part of the chromium oxide is reduced by hydrogen at the vapor phase epitaxy reactor temperatures to form chromium, which dopes the growing epitaxial layer. Resistivities of 10.sup.8 ohm-cm were reported for layers containing one ppm chromium, as long as the n-type background level was less than 5.times.10.sup.15 cm.sup.-3. The chromyl chloride process has several disadvantages, most of which can be attritubed to its oxygen content. The high resistivity layers formed in this way are inevitably doped with oxygen as well as chromium, and this may not be desirable. Various parts of the vapor phase epitial system become coated with chromium oxides which can unintentionally contaminate the growing layers. In addition, the precise amount of chromium incorporated into the growing layer is not easily controlled. Iron doping has been used chiefly with low temperature growth processes, such as the AsCl.sub.3 --N.sub.2 --Ga system. However, the resistivities obtained with iron are poorer (approximately 10.sup.5 ohm-cm) than those obtained with chromium doping.